Probing the Electronic Structure and Chemical Bonding of Uranium

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Probing the Electronic Structure and Chemical Bonding of Uranium Nitride Complexes of NU-XO (X=C, N, O) Jianwei Qin, Peng Zhang, Zhen Pu, Yin Hu, Ping Zhang, Maobing Shuai, and Shu-Xian Hu J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b02923 • Publication Date (Web): 18 Jul 2019 Downloaded from pubs.acs.org on July 20, 2019

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The Journal of Physical Chemistry

Probing the Electronic Structure and Chemical Bonding of Uranium Nitride Complexes of NU-XO (X=C, N, O) Jian-Wei Qin,a Peng Zhang,b Zhen Pu,c Yin Hu,a Ping Zhang,d Mao-Bing Shuai,*c Shu-Xian Hu*b a

Science and Technology on Surface Physics and Chemistry Laboratory, Mianyang 621908, China b Beijing Computational Science Research Center, Beijing 100193, China c Institute of Materials, China Academy of Engineering Physics, Mianyang 621907, China d Institute of Applied Physics and Computational Mathematics, Beijing 100088, China * Corresponding author. E-mail addresses: [email protected] (Shu-Xian Hu), [email protected] (Mao-Bing Shuai)

Abstract: Uranium (Ⅲ) compounds are very reactive and exhibit a broad range of chemical-bonding tendencies owing to the spatially diffused valence orbitals of uranium. A systematic study on geometries, electronic structures, and chemical bonding of NU-XO (X=C, N, O) is performed via using relativistic quantum chemistry approaches. The NU-CO and NU-NO complexes have an end-on structure, i.e. (NU)(1-CO) and (NU)(1-NO) while the NUOO adopts a side-on ((NU)(2-O2)) structure. The electronic structure analysis show that UN behaves efficient activation reactivity to those molecules, especially to NO and O2, due to the significant U 7s/5f  XO 2π* electron transfer. Thus, the oxidation state of U is +V with dianion ligand NO2- and O22- in NU-NO and NU-OO, respectively. Instead, U retains its usual +III oxidation state in NU-CO with a neutral CO ligand. The significant stability of the NU-XO (X=C, N, O) is determined by the covalent U–X bonding which contains both X  U -, π- donation from the X lone pair, and U 5f  XO 2π* back donation contributions. The significant back donation to the antibonding X–O 2π* orbital results in the obviously weakening of the X–O bonding.

1. Introduction Uranium mononitride, UN, has been proposed to be a suitable material in the nuclear energy system high fissile atom density and high thermal conductivity.1-2 However, owing to the spatially distributed 5f6d7s valence orbitals of uranium3-4 and the high redox capacity of the +3 oxidation state5-6, these uranium (Ⅲ) compounds-especially on its surface containing unsaturated low valent uranium, are very reactive and exhibit a broad range of chemical-bonding tendencies7. .Consequently, it is liable to form the uranium aerosol particles as the binding process of these uranium compounds to the oxygen-containing ligands occurred readily during their production and disposure in air, which will affect the environment and health security for its radioactivity and toxicity. 8 Therefore, it is very essential to get more insights in understanding the bonding process between the uranium nitride molecules and other species because of the potential scientific implications for preparing nuclear fuels and processing nuclear wastes.9 Moreover, uranium nitride chemistry is also an important supplement to reach a deeper understanding of 5f orbital participation in multiple bonding and covalency in actinide−ligand bonds. But in fact, the investigation of uranium nitride chemistry remains much less developed than other uranium oxide counterparts.10-11 As mentioned before, investigation about the geometry and electronic structure of simple actinide-containing molecules could provide knowledge about the uranium-mediated small-molecule activation chemistry. Therefore, a variety of typical prototype molecules containing bonding between uranium and other main group elements have been characterized and studied by experimental and theoretical calculations before.12-14 While U(VI)≡O bonds like 1

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those in uranyl are common and have been widely studied8, 15-16, fewer are known about the bonds between U and N from the literature. The first uranium nitride molecule was isolated in argon matrix by Green, and they also found some species labeled X-UN, where X represents the probable presence of one or more additional argon atoms.17 After that, Andrews and his collaborators investigated a series of polyatomic molecules NUX (X=O, N, NH, F) using matrix isolation technology and density functional theory (DFT) methods18-21. The occurrences of the migratory insertion reactions in their investigations, which were caused by the invocation of 5f orbitals for bonding and the polarizing nature of the uranium atoms12, confirmed the potential application of uranium in the small molecules activation domain. An identically triple N≡U bond is found in all these complexes, and the remained valence electrons on uranium can lead to the bonding with other species. Wang has identified the existence of NU(NN)1-6 in the cryogenic N2 matrix with weak NU-NN ligand interaction based on the slight red-shift of U-N stretching in the experiment.22-23 In fact the formation of these adducts is analogous to the five-coordinate homoleptic carbonyl UO2+(CO)5 and UO2+(RG)5 complexes detected in other gas phase experiments.24-25 Besides these UN-based small molecules in the gas phase mentioned above, a whole string of macro terminal uranium nitride complexes have been prepared by Liddle’s group,26 whose work have extensively developed the knowledge of the uranium nitride chemistry. However, the coordination of other moieties to the central uranium atom in these studies hardly indicate the activation of the UN to the ligands. Therefore, whether the splitting of the bonds in the small molecules, especially for the oxygen-containing molecules, can be achieved like that caused by bare uranium atoms and how the bonding between UN and other moieties behave need to be addressed. Meanwhile a thorough understanding of the interaction and bonding behavior between the UN and the common oxygen-containing species is able to provide fundamental knowledge to understand the short-term reactivity of [UN]n as a ceramic nuclear fuel and its long-term stability. There are two principle challenges for computational uranium chemistry: (i) the complexity of electronic states caused by the energetic near-degeneracy but radial separation of the U 5f, 6d, and 7s valence shells, and (ii) the present of remarkable relativistic effects in the valence shells of U in relatively low oxidation state, including scalar relativistic (SR) and spin−orbit coupling (SOC) effects. In this work, we have carried out theoretical studies on the nature of the geometries and the electronic structures of the NU-XO (X=C, N, O) complexes using various computational chemistry methods. Both SR and SOC DFT methods were used for these systems. The most stable structures for NU-XO from X=C to O are determined, and the dependence of the structure on the oxidation state of the metal atoms is investigated. Finally, the trend of chemical bonding between UN and XO is studied.

2. Computational methods Quantum chemical calculations included both density functional and high-level wave function-based methods. Firstly, several different density functionals, including the pure-GGA (PBE)27, hybrid-GGA (B3LYP)28-29 and hybrid meta-GGA (M06) methods30 have been employed to test the dependence of the computed results on the density functional employed, which gave similar results, so that only results obtained at the B3LYP level are presented herein. Geometry optimizations were performed using the B3LYP density functional with broken symmetry and the def2TZVP basis set31-32 for C, N, O atoms, while the scalar-relativistic small core SDD pseudopotential with 32-valenceelectrons was used for uranium associated with Gaussian valence basis set33-34. While this level of theory has been used previously with much success for the systems studied herein.35-36 Then, time dependent theory DFT (TDDFT)37 at B3LYP level were performed on the ground state structures of NU-XO to check the electron excitation energies and to ensure the stability of wavefunction of the open-shell electronic configurations for the ground states. All density functional theory (DFT) calculations were performed with the Gaussian 09 software package.38 All reported energies were obtained by combining the electronic energies with zero-point vibrational energy corrections. To study the electronic structure in further detail, complete active space self-consistent field calculations with corrections from second-order perturbation theory (CASSCF/CASPT2)39-40 were performed. And in order to quantitatively account for the relatively energy of different oxidation states of uranium, state-averaged CASSCF 2


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calculations41 based on the selection of active spaces were carried out to generate the wavefunctions for the ground and singlet- and triplet- excited states, which ensures that the results from DFT calculations are reliable. Scalar relativistic effects were taken into account through the use of the Douglas−Kroll−Hess (DKH) Hamiltonian42-43, and relativistic all-electron ANO-RCC-VDZP basis sets were employed with the following contractions: 8s7p5d3f1g for uranium44, and 3s2p1d for carbon, nitrogen and oxygen45. Multistate complete active space second-order perturbation theory (MS-CASPT2) calculations46 were performed as implemented in the Molcas 8.0 software package47-48. Cholesky decomposition49 was used in combination with local exchange (LK) screening50 to reduce the cost of the two electron integrals. The details are available in Tables S7-S10 and Figures S3-S5 in ESI. Finally, CCSD(T) calculations51 were performed by using def2-TZVP basis set and cc-pVQZ basis sets for C, N,O, combined with SDD, ECP60MWB_ANO and cc-PVQZ-PP for uranium,44, 52-55 calibrating the relative electronic energy for NU-XO with different oxidation state of uranium at a higher level of theory as implemented in the Molpro2017 software package56. Thus, only the CCSD(T) energies for the first two oxidation states of uranium in each NU-XO complex have been compared, which confirms the reliability of those results from DFT calculations. Further chemical bonding analyses were performed on the B3LYP/ with ADF 201757 at the B3LYP level. Slatertype basis sets of valence triple-ζ plus two polarization functions (TZ2P) with a frozen [1s2–5d10] shell for U atom and the frozen [1s2] shell for C, N, O were used58. Energy decomposition analyses (EDA)59-60 and combined extended transition state (ETS) with the natural orbitals for Chemical Valence (NOCV) theory61 were carried out to assess different bonding orbital contributions to the total bonding energies. In addition, geometric optimizations were also carried out at PBE/TZ2P level with specific occupation number for different electronic states of NU-XO to compare with the results obtained with Gaussian. The scalar relativistic (SR) effects were taken into account by the zero-order regular approximation (ZORA)62-63, then single-point energy calculations were performed with inclusion of the SO coupling effects via the SO-ZORA for the geometries optimized at the SR-ZORA level.

3. Results and discussion 3.1 Geometry structure Several density functional methods were employed on the geometry optimization for different state of the NU-XO (X=CO, NO, OO) complexes; detailed in Table 1. The computed ground state structures at B3LYP/def-TZVP/SDD level are shown in Table 2 and Figure 1. Before we present the theoretical results, let us summarize what have known experimentally on the uranium nitride diatomic species. The UN-N2 and UN-O2 were previously produced from cocondensation of laser-ablated metal atoms and electrons with nitrogen or dioxygen in excess argon, and were identified by infrared absorption spectroscopy. On the basis of 15N isotopic substitution experiments, the uraniumnitrogen stretching mode was observed at 1000.9 cm-1 in solid argon17 and having intense doublet of bands at 890.5 cm-1 and 878.2 cm-1 in solid dinitrogen. 22 UN. Morse compared the resonant two-photon ionization spectroscopy of jet-cooled UN molecules and concluded that the ground state of the molecule derives from a U3+ ions in its 7s15f2 atomic configuration.64 On the basis of several theoretical calculations, the UN was determined to have a bond length of ~1.75 Å with ground state of 4.21 Consistently, current B3LYP calculations show that the U-N bond length of 1.748 Å and its valence electronic configuration of s1f2. In the ground state of UN, the nature of bonding can be explained by molecular orbitals, as the fully occupied 12 and 14 bonding orbitals and the singly occupied 7s-base and 5f-base non-bonding molecular orbitals, giving a U≡N triple bond and indicating the +III oxidation state of U. To understand the inherent chemical interaction of uranium nitride, we analyzed the Kohn–Sham molecular orbitals (MOs) of the UN molecules and their MO energy levels with correlation to the atomic orbitals (AOs). The energy levels of the frontier 5f-, 6d- and 7sbased MOs of the UN moiety with Cv symmetry together with the corresponding MO iso-surfaces are shown in Fig. S1. Based on group theory, the seven U 5f orbitals in UN with Cv symmetry transform as f, fπ, f, f for m= 0, ±1, 3



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±2, ±3, respectively. Because of the U3+ (5f27s1) configuration, U valence AOs mix strongly due to the relativistic effects, and the chemical bonding pattern indicates significant 5f6d7s hybridization. However, because of the much more extended radial distribution and better symmetry match, because of the much more extended radial distribution of the U 6d AOs that favor large orbital overlap and thus leads to the stronger orbital interactions between the degenerate U 6dxz/6dyz and the N 2px/2py AOs, they lead to the wide energy separation between the fully occupied double degenerate 1π bonding orbitals and its corresponding antibonding MOs, which are virtually occupied in higher energy levels not shown in Figure 2 but labeled as 5π in Figure S1. The 2 MO is mainly contributed from U 7s AO, which is localized just above the bonding orbital 1 and is not shown in the energy level scheme (Figure S1). In contrast, the 5f manifold spans a band that is energetically less than the 6d one due to the much more contracted U 5f shell, similar to the 4f vs 5d bonding pattern in the [LnCO] species65-66. The significant bonding-orbital of 1 composed mainly from the interactions between U 5fz3/6dz2 AOs and the N 2pz AO, combining with the degenerated 1π orbital interactions produces to the U≡N triple bond, which significantly stabilize of the UN molecule owing to substantial energetic release. The orbital interactions within the UN framework can be approximately classified as σ-, π-, and δ-type bonding and antibonding pairs as well. Within a simple MO approximation, the seven U 5f orbitals form one double-degenerate non-bonding 1δ MOs, and two double-degenerate 1ϕ and 2π anti-bonding orbitals, and one 3-antibonding orbital, respectively. NU-CO. The ground state UN-CO has a bent structure with a UNC of 130.2 °. The U–N bond length increases to 1.756 Å due to the coordination by CO ligand and the C–O distance is elongated by more than 0.03 Å indicating that the carbon–oxygen triple bond is weakened because of coordination to UN. As well-known, the π back donation was seen as the primary interaction in forming the M–CO bond67. We now notice that, besides this π bond, a CO  donation can also play a role. The bonding between NU and CO fragments which is relatively simple can be interpreted through fragment molecular orbital (FMO) analysis. Fig. 2 shows the energy levels of the [NU-CO] species in Cs symmetry and their correlation to the levels of NU and CO fragments, and Fig. 3 gives the contours of NU-CO frontier orbitals, from HOMO-5 to LUMO+4. For the NU fragment, the N 2p-based MOs 1π and 1 transform into group orbitals 2a″+3a′+ 4a′ of NU-CO, respectively. The major orbital interaction contribution to the bonding comes from the direct overlap of the δ, π-type MOs (1δ and 2π) of NU and the C 2p-derived antibonding MO (π*) of CO, as shown in the blur colored lines in Fig. 4, whereas 1ϕ is independent of orbital interaction. Consequently, the U 5f-derived non-bonding orbital lie above NU bonding orbital manifolds. The ϕ-type highest singly occupied orbital (SOMO) is mainly a 5f6d hybrid of U and is a weak non-bonding orbital between U and CO. The two π-type orbitals (SOMO-1 and SOMO-2) are dative bonding orbitals between the U 5f6d and the 2π* antibonding orbital of CO, representing the metal-to-ligand back donation. The charge of carbon is -0.20|e| gaining 0.06|e| negative charge compared to that of CO because of metal back donation (see Table S5). The HOMO-3 (1a') orbital exerts  donation from the CO lone-pair of the CO to the UN. The  donation and π back-donation together create the U–C bond, and the latter concomitantly weakens the strong carbon–oxygen bond and thus activates the CO molecule. NU-NO. As the combination of UN with NO molecule, the U−N bond length decreases from 1.756 Å in the NU-CO to 1.744 Å in NU-NO, and the stretching frequency slightly increases from 999 cm-1 in NU-CO to 1022 cm-1 in NUNO. This is implying a stronger bond along the series due to the depopulation of the non-bonding f orbitals on the U≡N bond, which can be found in previous studies for other systems68. The ground state UN-NO also has a bent structure with a NUN of 142.5 °, in agreement with previous experimental and theoretical results22. Jahn-Teller dynamical instability is responsible for their non-totally symmetrical conformation.69 The N-O bond length of 1.229 Å in coordinated NO is longer than that of 1.146 Å in isolated NO molecule. The significant elongation implies the nominal N=O double bond reduced to a N-O single bond, according to the N=O bond covalent radii of Pyykkö.70 4


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These distances are consistent with the bonding analysis detailed below. The bonding interaction between NU fragment and NO fragment shown in Fig. 4 shows that there are significant orbital interactions between two fragments, dominated by π* MO of NO and U 5f/6d orbitals, leading to the stabilization of π* MOs and destabilization of U 5f/6d-derived MOs in NU-NO. The 2a″, 3a′ and 4a′ group orbitals are bonding orbitals from UN fragment, with the corresponding antibonding counterparts fully empty in higher energy levels; the 3D iso-surfaces are shown in Fig. 5. The two-orbital interaction between 1δ and 2π of UN and the π* group orbital of NO approximately forms bonding 5a′ and 3a″, which are reminiscent of the strong π-type bonding model. Here the singly occupied bonding 5a′ orbital can be viewed as the 2p-based NO- π* anti-bonding orbital gets stabilized upon interaction with UN fragment. The non-bonding 6a′ orbital with singly-occupation results in a large polarization splitting of 1ϕ MOs from UN upon coordinated by NO. The net consequence of these orbital interactions upon formation of the [NU-NO] species can be simply viewed as the π* orbitals of NO being stabilized energetically, leading to a tetravalent U4+ ion and a NO− ion in the complex. In addition, the strong π-type bonding orbital 3a″ represents a bonding feature of ligand-to-metal donation combined with metal-to-ligand back donation. The charge of nitrogen of NO increases from -0.09|e| in NO to -0.30|e| in UN-NO complex implying significant of charge-transfer from metal to ligand therefore elongates the bond length of NO (see Table S5), meanwhile a great polarization of UN induced by the positive charge gained on uranium is suggested from the charge analysis. Therefore, this wavefunction analyses clearly indicate that the U center has a (f1d0) configuration in NU-NO. Following the general rules for the determination of formal oxidation states, the end-on NU-NO can be classified as a U(+V) species. Thus, the increasing Coulomb interaction between U cation and N anion but less charge density localized on U-N moiety together shortens the U-N bond length but decrease the U–N bond strength, which is evident by the red-shift vibrational frequencies of 17 cm-1 for UN. NU-OO. As the reaction of UN with O2 molecules, the U−N bond length decreases to 1.737 Å in NU-OO, and the stretching frequency slightly increases to 1035 cm-1 in NU-OO. In contrast to the other two complexes which have an end-on structure with only the C or N atoms in the ligands binding towards uranium, like most of the other carbonyl71-72 and nitrosyl compounds,73 NU-OO adopts a side-on 2-coordinated structure with both the oxygen atoms binding to central uranium, which is similar to the uranium oxo structure in the gas phase revealed by Ricks.74 This can be attributed to the differences of the bonding process. The compound with O2 adducts could be designated as a typical peroxide form75-76 with the computed O-O bond length of 1.486 Å and the vibration frequency of 874 cm-1.To elucidate the metal−ligand interaction, [NU-OO] is viewed as forming from NU and O2. The MO energylevel correlation diagram of the [NU-OO] species is shown in Figure 6. According to the frontier MOs, the 1b2 and 1a2 orbitals represent π-bonding character, with corresponding antibonding 4b2 and 2a2 orbitals, respectively. The 3b2 MO has a character of non-bonding U 5f-derived orbital. The 1b2 MO with a large orbital energy stabilization shown in the plot reflects a strong bonding interaction between the central uranium and ligands; πg* of O2 ligand group orbitals overlaps with the corresponding 1δ fz(x2−y2) orbital of UN, forming the 1b2 bonding orbital and 4b2 antibonding orbital in [NU-OO]. Meantime, the πg* of O2 orbitals interact with the 1δ-2 fz orbital of UN with the formation of a pair of bonding and antibonding MOs, 1a2 and 2a2 in [NU-OO], however the splitting of this pair of bonding and antibonding orbitals accounts for less stabilization energy. Thus, the 1b2 orbital plays a key role in determining the stability of this complex. MOs with energy above the 3a1 level are responsible for the frontier vertical dissoc iation energy (VDE) bands could be observed experimentally. The splitting of U 5f orbitals upon interacting with the O2 π* MOs is consistent with the conventional crystal-field theory. The 3D isosurfaces of 12 frontier MOs of the [NU-OO] complex derived from one 7s, seven 5f and two 6d U AOs, and four 2p O AOs and six 2p N AOs are shown in Figure 7. These orbital contours are consistent with the orbital interaction analysis and indicate that interaction between UN and O2 is primarily based on 1b2 and 4b2 orbitals in the plane, while the other orbitals mainly belong to the intra-plane interactions, thus having relatively less contribution to the interlayer chemical bonding. 5



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3.2 Electronic structure As declared above, the TDDFT calculations have been performed on the ground state of each species. All excitation energies are positive and the first five excitation energies are listed in Table S11, which confirms the electronic character of ground states for the three NU-XO complexes. Thus, all the electronic structures and chemical bonding analyses below based on those ground states are reasonable. There are striking differences among those NUXO complex systems. In terms of their electronic structure, UN-NO has a ground state configuration of f1-24*3. Compared to the s1f2 electronic configuration in UN and that of 42*1 in NO, obviously, uranium ion transfers two electrons from U valence molecular orbitals (MOs) to the NO * MOs, giving an oxidation state of +V for U, which is largely ascribed to the direct U–N bonding interaction within the NU-NO in addition to the ionic interaction, proven by the orbital interactions between UN and XO fragments interpreted in next section. The neutral NU-OO molecule is predicted to have a C2v doublet (2B2) ground state. It is a U(V) species with the unpaired electron located in the metal center (U 5f1) with an electron configuration of (core) f1-24*4, which can be regarded as being formed via the electron-transfer from UN to O2. As stated by Bryantsev77 in a previous investigation about uranium oxo chemistry, the orbital interaction between frontier orbital of central uranium atom and the in-plane π and * orbital of O2 ligand is responsible for the bonding of uranyl superoxo complex due to its energy and symmetry proximity, which promotes the formation of a two-electron, three-center bond and contributes to a significant stabilization of the so-resulting U-O2 molecular orbitals of the side-on structure. However, carbon monoxide are considered as good Lewis base ligands,78 so the donation to the central uranium atoms from the lone pair electrons, which almost reside on the C atom of the ligands, is also an important contribution to the dative / bonds. The electrons in this -type bond mainly localized between U-C/N atoms and leads to the end-on structure, as suggested in the previous isoelectronic NUNN23 and [UOCO+]79 complexes. Nevertheless, another important point we need to bear in our mind is that the π-back donation from central uranium atom to unfilled π* orbital of ligands still dominates the bonding process in NUCO complexes. Aforementioned analysis based on MO diagram indicates that the unpaired spin is located on the fϕ-type MO of the U ion with a U (f1d0) center (Figure 6). Thus, the NU-OO complex can be described as [(NU)2+(O2)2-], a neutral uranium nitride-peroxide species with U in oxidation state +V. The structure and bonding of [(NU)2+(O2)2-] are reminiscent of those of actinide tetra-oxides (UO4 and PuO4)4, 80, which were computed to be a [(An+VO2)+(O2)-] species as well. As shown in Figures 6, the bonding between NU and O2 is dominated by the orbital interaction between the empty U 5fπ orbital of UN and the anti-bonding π* orbitals of O2, which results in increased U 5fπ orbital energy and decreased π* orbitals energy. Population analysis indicates that the NU moiety transfers about 0.78|e| to the O2 fragment. The transferred electrons occupy two O2 π* orbitals decreasing the strength of O2, indicated by the O-O stretching frequency of 874 cm-1 as typical O22- and elongating the bond length from 1.203 as a typical O=O double bond to 1.486 as a O22- moiety. This charge transfer interaction weakens the U≡N bonds of the UN fragment and causes a red-shift of the U-N stretching frequency of NU-OO with respect to that of UN. We note that despite the difficulty in the assignment of oxidation states of some actinide species, the present assignment of U(+V) oxidation state in NU-OO is clear via the computed spectroscopic and structural values. Lack of significant multireference features of these species also make the assignment of U(V) less vague. From the partially listed frontier MO profiles in Figures 3, 5 and 7 for [UN-XO] species, respectively, we found that the highest SOMOs are mainly composed of non-bonding U 5fϕ orbitals even for [NU-CO]. The SOMO-1 and SOMO-2 are mainly of NO πg* character in NU-NO, in contrast to that of U 5f6d character in NU-CO. The spinorbital coupling (SOC) splitting of the SR Kohn−Sham (KS) MO levels is shown in Figure 8 to qualitatively interpret the trend of the SO couplings in the [NU-XO] (X = C, N, O) series. With the inclusion of spin-orbit coupling, the doubly-degenerate spatial orbitals should split into two components in double group, e.g., a’  e1/2 + e1/2. However, it is obvious from Fig. S1 that the spin-orbit splitting effects of the occupied orbitals are negligible because they are 6


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mainly ligand-based orbitals. For the metal-based singly occupied molecular orbitals and virtual orbitals, the spinorbit coupling is relatively larger in NU-CO than in NU-OO, which is consistent with the well-known fact that the first-order spin-orbit effects is important for open-shell electronic configurations. By comparing the SOC splitting effects of these complexes, we can find that differential SO effects in the X np orbitals increase apparently from C to O, as expected. Orbitals of 6a’ in NU-CO and 4a1 in NU-OO showing interactions between U 7s and X 2p-type orbitals show increasingly stronger first-order SOC splitting as 2p-block element become more polarized, which agrees with the fact that larger polarization have differential SOC effects. SOC somehow affects the local electronic configuration of [UN-XO] species, it doesn’t affect the oxidation state of U or the chemical bonding in [UN-XO] species. Since spin-orbit coupling effects decrease as CO > NO > OO, we will therefore focus on the electronic structures calculated by only including scalar relativistic effects. 3.3 Stability of U Oxidation State in NU-XO: UIII, UV and UV. As shown in Figure 2 and Table S6, the 3a″ and 5a′ SOMOs of [NU-CO] primarily consist of the U 5f6d orbital localized entirely among the U and CO, due to the better match in orbital energies of the carbon 2p AOs leads to a much more covalent interaction between C and U than that between U and O. Therefore, the higher energy of the CO π* leads to an expected smaller HOMO-LUMO gap (6a′ to 7a″) in [NU-CO] (0.196 eV) as compared to that in NUOO (3b2 to 4a1) (0.298 eV), which implies that NU-CO will have very different optical properties as NU-OO. It can also be illustrated that tetravalent UIV is less stable than trivalent UⅢ because UIV with an extra electron would have to occupy the π* orbital with rather high energy, thus destabilizing the complexes. In comparison, the 3a″ HOMO of NU-NO, depicted in Figure 4 and Table S6, involves significant contributions from the U 5fδ AOs, showing a substantial decrease above 1 eV when NO gets closer to UN so that two electrons can be lost from these orbitals, resulting in UV being stable. Meanwhile the 5a′ SOMO localizes among the U, N and O, indicative of a dative covalent NO π*U 5f6d bonding. In contrast, the bonding between UN and O2 is dominated by the orbital interaction between the singly fδ and also higher energy empty dδ orbitals of UN and the in-plane πu* orbital of O2, which results in substantial stability of 1a2 and 1b2 doubly-occupied MOs. The 3b2 SOMO consisting of the U 5f6d hybrid AOs, acts as LUMO orbitals with large HOMO−LUMO gaps in NU-OO complexes, so that two electrons can be accepted from U AOs, featuring a UV in complex. 3.4 Trend of the X−O Bond in NU-XO (X = C, N, O). Along the series, we always also find the π donation molecular orbital, which destabilizes the U 6d-derived molecular orbitals due to repulsion between the electrons in XO 2 and U 6dδ. To make the back-donation efficient, a substantial occupation of the metal 5f orbital is needed, thus requiring a balance between the population of U 6d and electron promotion from the 7s/5f orbitals. Uranium cation have f3 ground configurations, a promotion from a 7s and 5f to a 6d is very difficult thus preventing the formation of the back-bonding in UN-XO molecule. Elongation of the X-O bond for the three complexes can be clearly seen as compared to free XO molecules. The largest variation of about 0.29 Å is found in UN-OO complex since the change-over from O2 to O22-, indicated as the triplet 3O2 in neutral-stabilized form with O-O distances around 1.2 Å, where the left-superscript indicates the spin multiplicity, to a closed-shell diatomic peroxide 1O22- with O-O distances around 1.5 Å. Comparably, the variation of 0.08 Å in UNNO complex with the formation of doublet 2NO2- anion from doublet 2NO neutral molecule along with uranium element changing oxidation state from U(III) in NU to U(V) in NU-NO. The relatively weak bonding and less chargetransfer between UN and CO fragments UN-CO complex causes only 0.04 Å elongation of C=O bond. These changes suggest that the strength of X-O is weakened after binding to the center uranium atom, which is verified by the chemical bonding analyses as shown in Table S5. Both the Mayer- and Weinberg- bond methods on top of the optimized ground structure at B3LYP/SDD level show that the bond order of OO dropped to ~0.9, and that of N=O decreased to 1.5, while the C=O bond reduced to 2.1, which shows clearly the activation ability of small molecules 7



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in the rank of O2> NO> CO due to the electron transfer to the * MOs. Noticed that the bond-lengths of the N-U in these complex although become a slightly shorter upon the XO approaching and the change become smaller as the ligand changed from CO to OO. The Mayer bonders of the NU complexes remain ~3. From the Mulliken charge analysis and the spin density analysis, it can be concluded that the charges have been less polarized on nitrogen atom within NU bond and more electrons has been depleted from uranium atom. That means the electrons transferring to the antibonding orbital of the coordinated ligands occurs to U cation center in the NU-XO series, which lead to the bond weakening and activation of the oxygen containing units. 3.5 Trend of the U−X Bond in NU-XO (X = C, N, O). The calculated bond orders based on various theoretical schemes and adiabatic stretching force constants in NUXO species are given in Tables 2 and 3, respectively. These results also give strong evidence that the U-X covalent interaction becomes stronger at the cost of weakening the covalent bonding between the central metal and the adjacent N in the UN. All methods give an increasing U–X bond order, taking Mayer bond order as an example from 1.0 for U-C to 1.6 for U-N and 1.8 for U-O. The U–X stretching frequencies show a similar trend, e.g. 324, 456 and 563 cm-1 for U-C, U-N and U-O asymmetric vibrational stretching, respectively. Table 3 provides results of the energy decomposition analysis (EDA) with SO coupling correction, showing the key role of the covalent chemical bonding interaction according to orbitals with different irreducible representations. E denotes the binding energy corresponding the reaction of NU + XO  NU-XO, based on the atomic fragments; Eint denotes the interaction energy between two fragments, which has three components; those are the Pauli repulsion, Epauli, the electrostatic interaction, Eelstat, and the stabilizing orbital interaction, Eorb. The preparation energies Eprep coming from either geometry distortion or electronic ionization, and its value equals the difference between the E term and the Eint term. The contribution to the relatively smaller preparation energy of NU-CO includes valence electron rearrangement energy from 7s15f2 in the NU fragment to 5f3 in the NU-CO as suggested by orbital composition analysis and NBO analysis, and the geometry distortion energy of two fragments. However, due to the electronic variation caused by the charge reassignment and the electrostatic interaction between the charged fragments, the values of Eprep in NUOO complex is significantly high. The orbital interaction energy is the predominant term among the two components of the Eint in all three species, which suggest a higher extent of covalent interaction between the fragments. Meanwhile, the orbital interaction term increases gradually as the ligands changed from CO to OO, indicating the more significant bonding effect between the NU moiety and the ligands because gradually increment of valence electrons in the ligands takes part in the bonding behavior. On the other hand, steric interactions with two component of Pauli repulsion and electrostatic in the NUCO, NUNO and NUOO species also gradually increase, due to the increasingly charged interaction. However, the percentage of Eelstat taking part in the interaction energies decreases along this series, whereas the orbital interactions becomes more important, denoting that the covalence play substantial role during NU and XO bonding. It is also shown that the contribution of the SO coupling effect to the total interaction energy of NU-CO is significant (21.1 eV), while the correction for NU-NO is 8.44 eV, close to that of 13.5 eV for NU-OO (Table 3). Accordingly, the relative energetic stability is not influenced by the SO correction. The intrinsic bonding mechanism behavior in terms of the major contributions to the orbital interactions between the NU fragment and XO fragment was analyzed with the energy decomposition analysisnatural orbitals for chemical valence (EDA-NOCV) method,59-61 which is a powerful tool to reveal detailed insight into chemical bonding.67, 81-82 According to the orbital interaction analyses, the two fragments were set as neutral UN with f 3 configuration and neutral XO in all the three NU-XO complexes. As listed in Table 4, the color code of the deformation densities suggests that the charge flows from red to blue. The orbital interaction is decomposed into three major terms, one -type and two π-type orbitals, which account for more than 97% energy composition totally. And the π-type interaction molecular orbitals in the NU-NO are 47.1 and 60.8 kcal/mol stronger than that in the NUCO. In contrast to the π-orbital interactions, the -orbital interaction in the NU-CO is almost identical to the NU-NO, 8


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suggesting that the different stability of these two species is mainly due to the energy contribution of π-type orbitals. Different from the major contribution from π-orbital interactions in NU-CO and NU-NO, the -orbital interactions in the NU-OO take the largest contribution to the interaction energy. 3.6 Trend of Molecular Orbitals of NU-XO (X = C, N, O). In Figure 8 the MO energy levels of all the NU-XO (X = C, N, O) species are shown. Accordingly, the eigenvalues and percent compositions for some of the frontier molecular orbitals in NU-XO (X = C, N, O) are listed in Table S6. The SOMOs in NU-CO are mainly U 5f-based orbitals with strong mixing of π*CO by a ratio of 64.5:29.7, and the decrement of U 5f contribution to 41.2, together with that of NO increasing to 57.1% in NU-NO. This ratio further reduces to 12.8:87.4 along the decrease of energy level lower than that of UN. Therefore, all three species indeed have non-negligible (d/f-p)π orbital interactions and they will play significant role in the electronic structures and properties of these compounds. Obviously, the HOMOs of NU-XO (X = C, N, O) species are predominantly formed by UN bonding  MOs. The metallic character of the LUMOs is consistent with the fact that these compounds can be electrochemically reduced into [NU-XO]− (X = C, N, O) ions.

4. Conclusions We have performed relativistic DFT and ab initio calculations on the NU-XO species. Our calculations provide the following guidance: (1) Qualitative analysis of the bonding of the NU-XO reveals the possibility of significant (f/d-p)π conjugation interactions within the non-linear molecules, which has been confirmed by the molecular orbitals analyses and EDA-NOCV analyses. It is shown that there is crucial π-back donation between the occupied 5f/6d orbitals of U atoms and the empty π* orbitals of the CO or NO, and π donation between the empty 5f/6d orbitals of U atoms and the occupied π* orbitals of the O22-. (2) Contrary to the formation of the NUV-NO and NUV-OO counterparts, the U(III) is the most stable oxidation for NU-CO species. Because of the large electrostatic repulsion of empty π* orbitals and small (f/d-p)π interaction between NU and CO fragments, UN cannot efficiently activate CO molecule. The [(NU)2+(O2)2-] is stabilized through strong orbital interaction and charge transfer, which has a very significant HOMO-LUMO energy gap. Surprisingly, uranium has adopted neither +IV nor +VI, but an intermediate oxidation state +V, which features a (NO)2- radical, due to NO is not oxidizing enough to oxide uranium to its highest oxidation state. Therefore, the UN is predicted to active NO and O2 molecules. Since searching for the efficient small molecule activator is still a challenging but interesting research topic, this contribution is intended to chart future directions in this chemistry with the hope that it becomes a more developed area of investigation. Supporting information Relative energies and bond-length of NU with different electronic configurations from PBE/TZ2P and B3LYP/TZ2P, selected geometric parameters of [NU-XO] complexes with different oxidation state obtained at B3LYP/def2TZVP(C,N,O)//SDD(U) level using Gaussian 09 and at PBE/ TZ2P level using ADF 2016, comparison of the structures and frequencies of UN and NU-XOs at various theory levels ranged from PBE/def2TZVP(C,N,O)//SDD(U) to M06/def2-TZVP(C,N,O)//SDD(U), charges and spin densities of NU, XO and [NU-XO] complexes at the level of B3LYP/def2-TZVP(C,N,O)//SDD(U) using Gaussian09 D01, orbital energies and percent compositions of some frontier MOs for UN and NU-XOs, state-averaged CASSCF calculation results for [NU-XO]s, the energy level scheme and corresponding MO contours of UN molecule and SR vs SO MO energy levels of [NUXO], natural valence orbitals of the [NU-XO] complex from CASSCF. (Tables S1-S10 and Figures S1-S5) Acknowledgment The work was supported by NSAF (No. U1630250 and U1530401), the Foundation of President of China Academy of Engineering Physics (No. YZJJSQ2017072), and the National Natural Science Foundation of China (No. 9



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Page 15 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Relative energies (kcal/mol), and electronic configurations of NUXO complexes using different methods. Species

NU-CO

NU-NO

NU-OO

1

Elec. conf.

ffδf

fπf



-*

f -

*3

OS of U

III

IV

V

State

4A‛‛

4A‛‛

3A

0.0

17.8

0.0

PBE/SR/TZ2P

0.0

13.0

PBE/SO/TZ2P

0.0

B3LYP/ def2-TZVP(C, N, O) + SDD(U)

ΔE

CCSD(T)/

(kcal/mol)

def2-TZVP(C, N, O) + SDD(U) CCSD(T)/ cc-pVQZ(C, N, O) + ECP60MWB_ANO(U) CCSD(T)/ cc-pVQZ (C,N,O)+cc-PVQZ -PP(U)

fπ f

s1 fδfδ

f

s1fδ

s1fδfδ

*2

*

*4

*3

*2

IV

V

IV

III

5A

2B

4B

6A

7.5

33.2

0.0

40.1

115.3

0.0

13.0

62.2

0.0

59.2

125.6

13.2

0.0

8.6

38.8

0.0

36.7

97.2

0.0

33.6

0.0

6.4

0.0

46.4

0.0

18.3

0.0

8.2

0.0

38.2

0.0

18.4

0.0

7.5

0.0

37.3

‛

‛‛

5A

III

‛

2

1

2

Table 2. Electronic configurations calculated geometries (bond-length in Å, bond angles in degrees), vibrational frequencies (unscaled, cm-1) and intensities (km/mol) of NU, XO and NU-XO complexes at the B3LYP level of thoery; NU-CO and NU-NO complexes have a Cs symmetry, while NU-OO has a C2v symmetry; B3LYP

NU

CO

NUCO

NO

NUNO

OO

NUOO

Elec. Conf.

s1f2

*0

f3-*0

*⊥1

f1-24*2

*⊥1*∥1

f1--*4

state

4-

1+

4A”

2

3A’

3

2B 2

UN

1.748

U(XO) XO

1.125

1.756

1.744

1.737

2.333

2.050

2.081

1.162

1.146

1.230

1.203

1.486

∠NUXO

130.2

142.5

159.1

∠UXO

175.9

174.8

169.1

UX

324 (5.0)

456 (39.6)

563 (130.9)

UX

265 (5.5)

205 (12.4)

439 (15.5)

999 (283.0)

1022 (307.5)

1035 (241.8)

Freq.

UN

1039 (208.4)

XO

2214(78.6)

1898(1342.6)

1977(39.7)

1423(253.2)

1636 (0.0)

874 (62.7)

Table 3. Energy decomposition analysis (EDA, kcal/mol) in terms of NU and XO fragments in NU-XO complexes. NU-CO (4A”)

NU-NO (3A’)

NU-O2 (2B2)

NU (4Π) f2fπ1

NU (4Π) f2fπ1

NU (4Π) f2fπ1

CO (1Σ+) σ2π4π*0

NO (2Π) σ2π2π*1

O2 (3Σ+) σ2π4π*2

ΔE

-41.73

-80.31

-135.87

ΔEprep

16.50

17.66

49.81

ΔEint

-58.23

-97.97

-185.68

Interacting fragments*

15



Page 16 of 23

ΔEint-SO

-37.13

-89.53

-175.2

ΔESteric

60.89

136.52

195.05

ΔEpauli

133.04

273.88

386.64

ΔEelstat

-72.15(40.5%)

-137.36 (36.9%)

-191.58 (33.5%)

ΔEorb

-106.18 (59.5%)

-234.49(63.1%)

-380.73 (66.5%)

* E, the binding energy corresponding the reaction of NU + XO  NU-XO; Eint, total interaction energy between the two deformed fragments; Epauli, Pauli repulsion between the two deformed fragments; Eelstat, electrostatic between the two deformed fragments; Eorb, stabilizing orbital interaction between the two deformed fragments; Eprep, preparation energies.

Table 4. Contours of deformation densities (contour value 0.002) from EDA-NOCV analysis, describing the density inflow (blue) and outflow (red) between the interacting fragments of NU-XO and their corresponding energy ΔEiorb (kcal.mol−1)

NUCO

Orbital type

σ

π

π

ΔEiorb

-22.6

-39.3

-41.5

-27.8

-86.4

-102.3

Orbital type

σ

π

σ

ΔEiorb

-33.2

-125.9

-192.5

3-D contour

ΔEiorb NUNO

NUO2

3-D contour

3-D contour

16


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NU-CO

4A"

(f3-π*0)

4A"

(f2-π*1)

NU-NO

3A'

(f1-π*3)

5A"

(f2-π*2)

5A'

(s1f2-π*1)

NU-O2

2B 2

(f1-π*4)

4B 1

(f2-π*3)

6A

2 (s

1f2-π*2)

Figure 1. Computed structures for NU-XO isomers at B3LYP/def2-TZVP(C,N,O)//SDD(U) level of theory. The bond lengths are given in Å.

17



Page 18 of 23

Figure 2. The molecular orbital energy level schemes obtained at PBE/TZ2P level, illustrating the correlations of the MOs of the NUCO complexes in terms of NU with CO.

9a' U-5f6d -2.479

5a'' U-5fϕ -2.773

8a' U-5fδ -2.890

4a'' U-5fϕ 5fδ5fπ -2.950

7a' U-7s5fπ6d -3.311

SOMO: 6a' U-5fϕ -3.507

SOMO: 5a' U-5f6d/CO-π//* -3.945

SOMO: 3a''

4a' NU-σ -5.469

3a' NU-π//* -6.125

2a''

2a' U-7s6d/CO-σ -9.472

1a' CO-π// -11.647

1a''

U-5f6d/CO-π* -4.078

NU-π* -6.330

CO-π -11.715 18


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Figure 3. Frontier Kohn-sham molecule orbitals of the NU-CO complex. The value of the contour isovalues is 0.03 a.u. Only the alpha spin orbitals are presented, with color of blue and red for the phases of the occupied orbitals in contrast to that of yellow and cyan for the phases of the virtual orbitals.

Figure 4. The molecular orbital energy level schemes obtained at PBE/TZ2P level, illustrating the correlations of the MOs of the NU-NO complexes in terms of NU with NO.

6a'' U-5fπ -2.367

5a'' U-5fϕ -2.895

8a' U-5fδ -2.985

4a'' U-5fδ -3.061

7a' U-7s5fπ -3.390

6a' SOMO: U-5fϕ -3.622

19



5a' SOMO: NO-π* -4.477

1a'' NO-π -11.326

3a'' UN-π -4.921

4a' NU-σ -5.641

3a' NU-π// -6.410

Page 20 of 23

2a'' NU-π -6.505

2a' U-7s6d/NO-σ -10.040

1a' NO-π// -12.028

Figure 5. Frontier Kohn-sham molecule orbitals of the NU-NO complex. The value of the contour isovalues is 0.03 a.u. Only the alpha spin orbitals are presented, with color of blue and red for the phases of the occupied orbitals in contrast to that of yellow and cyan for the phases of the virtual orbitals.

Figure 6. The molecular orbital energy level schemes obtained at PBE/TZ2P level, illustrating the correlations of the MOs of the NU-OO complexes in terms of NU with O2.

20


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7a1 U-5fσ 0.161

4b2 U-5fπ -0.556

3a2 U-6d -1.532

6a1 U-6d -1.791

4b1 U -5fπ -1.979

3b1 U-5fϕ -2.547

2a2 U - 5fδ -2.590

5a1 U-5fδ7s -2.636

4a1 U-7s5fδ6d -3.073

3b2 SOMO: U-5fϕ -3.371

3a1 NU-σ -5.352

2b2 NU-π// -6.385

1a2

2b1 NU-π -6.463

2a1 O2 -π// -8.944

1 b1

O2-π* -6.417

1b2 O2-π//* -7.621

1a1 O2 -σ -10.788

O2 -π -9.989

Figure 7. Frontier Kohn-sham molecule orbitals of the NU-OO complex. The value of the contour isovalues is 0.03 a.u. Only the alpha spin orbitals are presented, with color of blue and red for the phases of the occupied orbitals in contrast to that of yellow and cyan for the phases of the virtual orbitals.

21



Page 22 of 23

Figure 8. Scalar relativistic KS-B3LYP molecular orbital energy levels of free NU-XO species for X = C, N, O.

22


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Table of Contents

The geometrical and electronic structures of NU-XO (X = C, N, O) complexes, the oxidation features of uranium, and the prediction of NU moiety to active CO, NO and O2 molecules are explored and characterized using relativistic quantum chemistry. This conclusion is potential to provide fundamental understanding of uranium compounds in this area of small molecule activation chemistry with the hope that it becomes a more developed area of investigation.

23


Probing the Electronic Structure and Chemical Bonding of Uranium

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